The contacts between mitochondria and endoplasmic reticulum (ER) play an important function in cell metabolism – they secure a direct calcium transmission from ER to the mitochondria. Upon opening of the inositol 1,4,5-triphosphate (IP3)-gated channels of the ER, the mitochondrial surface becomes exposed to a higher concentration of Ca2+ than that in the bulk cytosol. This enables the uptake of calcium by mitochondria via low affinity calcium uniporter [1]. In this study we tried to find answers to the following three questions: How do the contacts between mitochondria and the ER look like? Is the mitochondrial permeability transition pore (PTP) located in the proximity of these contacts? Is PTP the main target for calcium originating from the ER during stimulation of the cell.

For this purpose we studied protein components of mitochondria-associated membranes (MAM fraction) which can be a "junction bridge" between ER and mitochondria. Then, we investigated the roles of the voltage-dependent anion channel (VDAC) of the outer mitochondrial membrane and of the adenine nucleotide translocase (ANT), components of the PTP, in modulating mitochondrial calcium response. We overexpressed VDAC and three isoforms of ANT (ANT-1, ANT-2, ANT-3) in HeLa cells and analyzed calcium homeostasis, mitochondrial membrane potential and structure of mitochondria and the endoplasmic reticulum (ER). In our a paper [2] we have proposed that VDAC is a key determinant of Ca2+ permeability at ER-mitochondria contacts and is thus responsible for exposing calcium uniporter of the inner mitochondrial membrane to the large [Ca2+] gradients needed for rapidly accumulating Ca2+ in mitochondria upon cell stimulation. Based on this and on our recent results, we propose that VDAC increases calcium permeability of the ER-mitochondria contact sites. On the other hand, we observe large, cyclosporin A sensitive, reduction of mitochondrial calcium uptake in ANT-1 and ANT-3 transfected cells. The cytosolic calcium response after agonist stimulation is as in control cells. Moreover, overexpression of ANT-1 and ANT-3 (but not ANT-2) induces mitochondrial fragmentation. This and other our results could clarify the role of a larger complex including VDAC, ANT, cyclophilin D, the peripheral benzodiazepine receptor and members of the Bcl-2 family in the interaction between mitochondria and ER, and the possibility that some of these proteins are a part of the molecular machinery docking mitochondria to Ca2+ stores.